Hypoxia and ocean acidification (OA) are amongst the major environmental threats to marine ecosystems worldwide. Biofilms, the signpost to guide larval settlement of many benthic invertebrates, are known to be responsive to environmental changes and thus can become the crucial factor for the response of benthic invertebrate communities. This study aimed at investigating the individual and interactive effects of hypoxia and OA on biofilms and the subsequent effects on larval settlement. Biofilms collected from two sites (clean, hypoxic) were treated with a factorial design of low dissolved oxygen and/or low pH conditions in microcosms and the bacterial cell density and viability (by LIVE/DEAD® cell viability assays) were analyzed. Larval settlement preference was tested with the marine invertebrate, Crepidula onyx. The total bacterial cell densities of biofilms of the hypoxia and hypoxia and OA combination treatment were lower than that of the control biofilms for both sites. There was generally no significant difference in cell viability among control and different treatments for both sites. While the larval settlement rate on hypoxia and hypoxia and OA combination treated biofilms was significantly lower. In conclusion, this study revealed that hypoxia and OA are likely to affect larval settlement by alteration of biofilms, and this may lead to alterations in future coastal communities.

Submitted by Haroudo Xavier Filho (haroudo.xavierfo@ufpe.br) on 2016-10-07T14:36:45Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertação_ThaynaCavalcanti_BiologiaAnimal_2016.pdf: 2263756 bytes, checksum: 798ca1c12e15256db092df9fe703fdaa (MD5) / Made available in DSpace on 2016-10-07T14:36:45Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertação_ThaynaCavalcanti_BiologiaAnimal_2016.pdf: 2263756 bytes, checksum: 798ca1c12e15256db092df9fe703fdaa (MD5) Previous issue date: 2016-07-01 / Briozoários são organismos sésseis e coloniais, que dependem de uma superfície firme para assentamento larval e crescimento da colônia. Entre os diferentes substratos disponíveis para briozoários, as esponjas podem oferecer uma superfície favorável, trazendo muitas vantagens como a presença de compostos químicos que inibem a predação. Este estudo verificou a presença de briozoários em seis espécies de esponjas, com finalidade de avaliar a abundância e riqueza desses briozoários em diferentes esponjas. Três espécimes das esponjas, Amphimedon compressa Duchassaing & Michelotti, 1864, Amphimedon viridis Duchassaing & Michelotti, 1864, Desmapsamma anchorata (Carter, 1882), Dysidea etheria de Laubenfels, 1936, Haliclona implexiformis (Hechtel, 1965) e Tedania ignis (Duchassaing & Michelotti, 1864) foram coletadas mensalmente entre setembro de 2014 e fevereiro de 2016, em Pontas de Pedra, Pernambuco, Brasil. Os briozoários encontrados foram identificados até o menor nível taxonômico possível e quantificados. Um total de 324 espécimes de esponjas foi analisado, no qual destas 88 apresentavam briozoários em sua superfície (27%). Onze espécies de briozoários pertencentes à Classe Gymnolaemata foram encontrados nas esponjas, sendo quatro pertencentes à Ordem Ctenostomata, Amathia distans Busk, 1886, Amathia verticillata (delle Chiaje, 1822), Amathia vidovici Heller,1867 e Nolella stipata Gosse, 1855, sete da Ordem Cheilostomata, Beania klugei Cook, 1968, Catenicella uberrima (Harmer, 1957), Caulibugula dendograpta (Waters, 1913), Licornia sp., Savignyella lafontii (Audoin, 1826), Synnotum aegyptiacum Canu & Bassler, 1928 e Thalamoporella floridana Osburn, 1940. Briozoários foram abundantes nas esponjas Te. ignis e De. anchorata e pouco frequentes em Ap. compressa e Ap. viridis. Desmapsamma anchorata e Te. ignis apresentaram a maior riqueza de espécies (nove espécies em cada esponja), seguida por Dy. etheria (sete espécies). Uma baixa riqueza de espécies foi observada em Ap. compressa, com apenas três espécies de briozoários, Ap. viridis com quatro espécies, e H. implexiformis com cinco espécies. Apenas o briozoário N. stipata foi encontrado em todas as espécies de esponjas, enquanto que At. distans e At. vidovici não foram encontradas apenas em Ap. compressa. Uma maior riqueza e abundancia de briozoários foram encontradas em De. anchorata e Te. ignis, que apresentam superfície lisa e aveludada, e lisa e vilosa, respectivamente. Por outro lado, superfície lisa também é característica das espécies Ap. compressa e Ap. viridis, que apresentaram a menor frequência e diversidade de briozoários. Adicionalmente, as esponjas De. anchorata e Dy. etheria que apresentam superfície lisa e conulosa, respectivamente, compartilharam grande parte das espécies encontradas. A presença de metabólitos secundários nas esponjas do gênero Amphimedon que apresentam toxicidade já descrita na literatura, pode ter influenciado na ocorrência dos briozoários. Enquanto que as esponjas Te. Ignis, De. anchorata e Dy. Etheria, que apresentaram uma grande abundância de briozoários, podem ter provido um microhabitat adequado para os briozoários da região. O padrão temporal de ocorrência dos briozoários nas esponjas durante os 18 meses de coleta foi aleatório. O presente trabalho permite identificar alguns padrões da ocorrência dos briozoários, relacionado a composição química das esponjas e sua posição no substrato. / Bryozoans comprise sessile, colonial organisms that require a hard surface for settlement and growth. Among different substrata for bryozoans, sponges may provide suitable substrata, with advantages such as presence of compounds against predators. This study analyses the bryozoan community on six sponges species throughout 18 months, to evaluate the presence of bryozoans on its surface. Three specimens of each sponges, Amphimedon compressa Duchassaing & Michelotti, 1864, Amphimedon viridis Duchassaing & Michelotti, 1864, Desmapsamma anchorata (Carter, 1882), Dysidea etheria de Laubenfels, 1936, Haliclona implexiformis (Hechtel, 1965) and Tedania ignis (Duchassaing & Michelotti, 1864), were taken monthly betweem September 2014 to February 2016, in Pontas de Pedra, Pernambuco State, Brazil. Bryozoans were identified to the lowest taxonomic level, and quantified. Total of 324 specimens of sponges were analysed, 88 from those were found bryozoans on its surface (27%). Eleven gymnolaemate bryozoans were found on sponges being four of the Order Ctenostomata, Amathia distans Busk, 1886, Amathia verticillata (delle Chiaje, 1822), Amathia vidovici Heller,1867 and Nolella stipata Gosse, 1855, and seven species Cheilostomata, Beania klugei Cook, 1968, Catenicella uberrima (Harmer, 1957), Caulibugula dendograpta (Waters, 1913), Licornia sp., Savignyella lafontii (Audoin, 1826), Synnotum aegyptiacum Canu & Bassler, 1928 and Thalamoporella floridana Osburn, 1940. Bryozoans were considered abundant on the sponges Te. ignis and De. anchorata, but few frequents on Ap. compressa and Ap. viridis. On De. anchorata and Te. ignis were found the highest bryozoan richness (9 species of bryozoan per sponge species), followed by Dy. etheria (7 bryozoan species). A low species richness was observed in Ap. compressa, Ap. viridis and H. implexiformis with respectively three, four and five species of bryozoans on their surface. Only N. stipata was found on the six sponge species, while At. distans and At. vidovici were not found only on Ap. compressa. Higher richness and abundance of bryozoans were found in De. anchorata and Te. ignis, with smooth and velvety surface, smooth and villous surface, respectively. Smooth surface is also characteristic of Ap. compressa and Ap. viridis, with the lowest frequency and diversity of bryozoans. Additionally, on sponges De. anchorata and Dy. etheria, with smooth and conulose surface respectively, were shared the majority of bryozoan species. We suggest the presence of secondary metabolites may have interfered the bryozoans on sponges of Amphimedon genus. Sponges Te. ignis, De. anchorata and Dy. etheria, have an abundance of bryozoans; thus, we suggest these sponges may allow a suitable substratum for the bryozoans. The temporal variation of bryozoans on sponges was random. In present work some patterns of occurrence of bryozoans are presented, with relation to the chemical composition and position of sponge.

Meiofaunal distribution and abundance were studied in Marion Lake, a small, shallow (8m maximum) oligotrophic lake in southwestern British Columbia. Experimental techniques were used to investigate the influence of food and predation on meiofaunal populations, and to estimate relative carbon -flow to all components (micro-, meio-, and macrofauna) of the zoobenthic community. In two years' sampling of over 50 species of meiofaunal rotifers, nematodes, copepods, cladocerans and halacarine mites, only a few species were abundant. Three depth zones were sampled (1.0, 2.5, 4.5m) and maximum densities occurred at 2.5m. Population densities of all groups were stable over the sampling period, fluctuating less than one order of magnitude annually. In culture studies, representative meiofaunal species had longer generation times and lower reproductive rates than expected. Attempts to use laboratory results to predict field population dynamics were generally unsuccessful, but led to clarification of reproductive parameter estimates. Experiments in the laboratory and in the field suggested that meiofaunal species are not food-limited. Predation on meiofauna is not heavy but might be significant for those species whose reproduction is suppressed by adverse temperatures. Radiotracer experiments indicated that carbon flow to the zoobenthic community from sediment microflora was partitioned approximately 12% to microfauna (ciliates), 12% to meiofauna, and 76% to macrofauna, while the contributions to zoobenthic biomass were 1%, 7% and 92% respectively. In related experiments, a common harpacticoid copepod species had a high assimilation efficiency but rapidly respired and excreted recently ingested carbon. Meiofaunal organisms are apparently not an important food source for higher trophic levels in Marion Lake but may play a significant role in stimulating microfloral production by their grazing activity. / Science, Faculty of / Zoology, Department of / Graduate

Benthic algal ecology and primary pathways of energy flow were considered on the Squamish River delta at the head of Howe Sound, a fjord-type estuary. The study elucidated the structure and function of major autotrophic components of the estuarine ecosystem. Benthic algae were investigated with regard to species composition and distribution and the capacity for energy conversion, input to the system and storage. Comparisons were made with existing information on the vascular plant component of the ecosystem. The benthic algal community was studied by regular field sampling of major macroalgae and microalgal associations with a monitoring of physical-chemical environmental factors. Presence of an alga in the estuary was a function of its osmoregulatory capabilities. Establishment and temporal-spatial distribution patterns were controlled by substrate-habitat preference and availability and the interaction of light, interspecies competition, desiccation, temperature and salinity, light being of greatest importance. Carex lyngbyei Hornem., the dominant vascular plant, had a significant effect on distribution of benthic algae through light restriction during Its summer growth period and action as a substrate during the winter. Total species diversity, biomass and distributional area of benthic algae were greatest at the latter period. The effect of ecosystem structure on function was investigated by analysis of energy flux through major benthic algal producers. Comparisons were made of the total amount of energy input attributable to benthic algae and vascular plants. The importance of an algal producer to energy flux•m⁻² was a function of either high primary productivity, photosynthetic efficiency and caloric content, or in the case of diatom dominated micro-algal associations, high caloric content alone. Distribution, reflecting the presence of suitable substrate-habitat, modified this pattern. Macroalgae having high energy input•m⁻² (Monoetroma oxyapermum (Kutz.) Doty, Pylaiella littoral-is (Lyngb.) Kjell.) were of minimum importance to total energy input. Two microalgal associations (Association E, diatom dominated, Association G, Ulothrix flaoca (Dill.) Thur. dominated), each with low energy input-m but with wide distribution and high photosynthetic efficiency and caloric content contributed a total of 8H% of available energy attributable to benthic algae. Benthic algae account for a maximum of ca. 7% of total energy input to the delta ecosystem compared to ca. 90$ by vascular plants and 3% by addition of organic matter. The majority of energy for the detrital based ecosystem comes from vascular plants and becomes available after a lag period allowing decomposition. Benthic algae are significant to the ecosystem as a readily available, continually present energy source requiring little or no breakdown for utilization and not for total energy input. Energy is available as either dissolved or particulate organic matter. Of the latter, ca. 49$ is removed to the estuary, 33% incorporated into the sediments of the delta and 18$ used by consumers in the delta ecosystem. / Science, Faculty of / Botany, Department of / Graduate

Information about the benthic macroinvertebrate populations along the eastern shore of Goshen Bay, Utah Lake, Utah, can be used, in part, to determine future management of the lake. In the project herein reported, cement artificial substrate samplers were used to sample two types of substrate: rubble and compacted calcareous tufa. Monthly samples were obtained from each type of substrate from March 1972 to May 1973. An amphipod, Hyalela azteca and a chironomid, Dicrotendipes fumidus were the dominant organisms in numbers and tiomass. Amphipod numbers were dependent upon the amount of algal standing crop. Elimination of this area could affect the trophic structure of the lake by effectively reducing the macroinvertebrate population of the lake.

No description available.

Benthic macroinvertebrates and physicochemical parameters were sampled monthly in Spring Lake, Florida from July, 1978, to June, 1979. Spring Lake is a slightly acidic, sand bottom lake located in the sandhill region of the Central Highlands. While submersed vegetation is not dense, the lake does contain an abundance of the endemic submersed plant Mayaca aubletii. The littoral zone is dominated by plants belonging to the genera Panicum, Nuphar, Hydrocotyle, Nymphaea, Satittaria, and Typha. The benthic macroinvertebrates collected consisted of 51 species; approximately 50 percent were in the family Chironomidae. The annual mean number of individuals was 947/m2. The mayfly Hexagenia munda Orlando was the most numerous species (18.4 percent of the annual mean); the Chironomidae was the most numerous family (31.6 percent of the annual mean). The annual mean value for the Simpson's Index was 0.25 while the annual mean value for the Shannon Index was 2.60.

Benthic macroinvertebrates and physicochemical parameters were monitored simultaneously with hydrilla in a central Florida lake. Changes in the benthos were described in relation to plant growth. Twenty-seven of the 54 taxa of benthic macroinvertebrates collected were members of the Family Chironomidae. Chironomids and oligochaetes numerically dominated the benthos, comprising of 82% of the individual collected. The greatest numbers of species and individuals were found during the winter when hydrilla biomass was low. Hydrilla biomass ranged from 0.385 kg/m2 in April, 1977 to 2.275 kg/m2 in October, 1977. Low numbers of species and individuals were collected from bottom sediments during summer and fall. Dissolved oxygen concentrations at the bottom were approximately 2.0 ppm during summer and fall and possibly limited benthic organisms. The annual means for the Shannon and Simpson Indices for the benthos were 1.92 and 0.36, respectively.

Les régions côtières des estuaires sont le siège de nombreuses activités anthropiques terrestres ou aquatiques comme l'agriculture, le transport maritime ou l'aquaculture. Ces différentes activités engendrent des facteurs de stress, communément appelés stresseurs, qui affectent la structure et le fonctionnement des communautés benthiques. Avec l'augmentation des activités humaines, deux scénarios principaux ont été notés : i) le chevauchement spatio-temporelle de plusieurs facteurs de stress, et ii) l'intensification de stresseurs uniques. Dans un premier temps, la superposition spatiotemporelle de plusieurs stresseurs peut mener à l'interaction entre ceux-ci, où la présence d'un stresseur accentue (synergie) ou, au contraire, atténue l'effet d'un autre (interaction antagoniste). Dans un deuxième temps, l'intensification de stresseurs uniques peut mener à l'apparition de points de bascule, correspondant à des réorganisations substantielles dans la structure et le fonctionnement d'un écosystème. L'interaction de stresseurs et les points de bascule ont de profondes implications sur les écosystèmes; ils sont souvent très difficiles à prévoir, et peuvent occasionner des changements importants au sein de communautés naturelles, à un point tel que le retour à l'état initial est parfois impossible. Ces deux concepts ont largement été étudiés de façon exclusive. Toutefois, très peu d'études ont cherché à combiner ces deux concepts. C'est à la lumière des activités humaines et de leurs impacts sur les écosystèmes aquatiques, notamment ceux de l'estuaire du Saint-Laurent, que se justifie mon projet doctoral. Plus précisément, l'objectif général de cette thèse est de comprendre l'effet de multiples stresseurs d'origine anthropique sur les écosystèmes aquatiques en intégrant différents compartiments biologiques (c.-à-d. individus, populations, communautés, écosystèmes). Cette thèse s'articule autour de deux objectifs principaux, divisés en cinq chapitres. Le premier objectif est de déterminer les scénarios d'interaction entre plusieurs stresseurs et leurs effets sur les communautés benthiques à l'aide d'approches expérimentales en laboratoire et in-situ (Chapitres 1 et 2). Le deuxième objectif est d'identifier des points de bascule lorsque les communautés sont exposées à différentes intensités et combinaisons de stresseurs à l'aide d'une revue de littérature et d'une expérience en laboratoire (Chapitres 3, 4 et 5). Mon premier chapitre s'est concentré sur l'effet individuel et combiné d'un enrichissement en nutriments, d'une variation de salinité et une augmentation de la température de l'eau sur la moule bleue, Mytilus spp., la telline de la Balthique, Limecola balthica, et sur le microphytobenthos. Pour répondre à ces objectifs, nous avons effectué une expérience de manipulation en laboratoire de trois mois où la mortalité, le contenu énergétique des tissus et la minéralisation des coquilles ont été évalués chez Mytilus spp. et chez L. balthica, ainsi que les pigments microphytobenthiques. La variation de salinité et l'enrichissement en nutriments, séparément, ont eu d'importants effets sur la mortalité, le niveau de minéralisation des coquilles, le contenu énergétique des tissus, et la biomasse microphytobenthique et leur effet variait dans le temps. Au contraire, l'augmentation de la température de l'eau n'a eu aucun effet. Ces résultats démontrent une prévalence d'interactions antagonistes pour les réponses évaluées et l'importance de considérer l'effet de stresseurs uniques et combinés dans le temps. Sur la même thématique, mon deuxième chapitre a cherché à caractériser in situ l'effet de vagues de chaleur et d'un enrichissement en nutriments sur les bancs de moules et l'écosystème qu'elles créent. Nous avons mené une expérience où la mortalité et le contenu énergétique chez Mytilus spp., l'activité microbienne, et la concentration sédimentaire en chlorophylle a et en phaeopigments ont été évalués au cours du temps. L'enrichissement en nutriments et les vagues de chaleur, de manière individuelle et combinée, ont eu des effets positifs et négatifs sur le contenu énergétique des tissus chez Mytilus spp., sur les phaeopigments ainsi que sur le ratio chlorophylle a/phaeopigments. En ce qui concerne les autres réponses d'intérêt, aucun effet n'a été observé, suggérant que les stresseurs influencent de manière différente les différents compartiments biologiques et écosystémiques étudiés. Ce chapitre démontre que l'effet individuel et combiné des stresseurs environnementaux diffère à travers le temps et illustre l'importance de combiner plusieurs approches expérimentales pour cerner la complexité derrière l'interaction entre stresseurs multiples. Pour mon troisième chapitre, à l'aide d'une revue de la documentation scientifique, nous avons (1) développé une perspective actuelle et historique sur les points de bascule dans les écosystèmes terrestres et aquatiques; (2) décrit l'effort de recherche dans différents habitats aquatiques; (3) exploré les résultats d'études expérimentales axés sur les points de bascule mesurés aux échelles individuelles, des populations, des communautés, et des services/fonctions écosystémiques dans un contexte de stresseurs uniques et multiples. Le nombre d'études portant sur les points de bascule augmente annuellement, mais très peu d'études ont pour objectif spécifique de les identifier (32.6%). Encore moins d'études examinent comment l'ajout d'un stresseur additionnel peut modifier un point de bascule. Aussi, plusieurs études s'intéressent à de multiples réponses, mais seulement 25% se concentrent sur de multiples échelles biologiques. Ces résultats démontrent que le concept de point de bascule est très actuel, mais nous avons identifié certaines pistes de solution afin de renouveler ce concept dans un contexte de stresseurs multiples. Dans le cadre de mon quatrième et cinquième chapitre, nous avons mené une expérience afin d'étudier les effets d'une intrusion sporadique d'eau salée le long d'un gradient d'enrichissement en nutriments sur une communauté d'eau douce composée de périphyton, de microorganismes hétérotrophes, et d'individus de moule zébrée Dreissena polymorpha. Nous cherchions à (1) caractériser l'effet individuel et combiné des deux stresseurs sur les réponses d'intérêts et identifier des interactions synergiques ou antagonistes; (2) identifier un point de bascule le long du gradient d'enrichissement en nutriment en absence et présence d'une intrusion sporadique d'eau salée; puis, (3) identifier l'effet de la présence d'un stress osmotique sur l'atteinte d'un point de bascule. Nous nous sommes intéressés à la mortalité (Chapitre 4) et au taux métabolique chez D. polymorpha, à l'activité microbienne, et au périphyton (chlorophylle a, phaeopigments) (Chapitre 5). Les deux stresseurs ont eu des effets individuels (c.-à-d. dominances) et combinés (c.-à-d. interactions) sur toutes les variables réponses, et nous avons observé un changement au niveau du type d'interaction le long du gradient. La présence d'un stress osmotique a eu pour effet de devancer le point de bascule pour la mortalité et de créer un point de bascule pour le taux métabolique chez D. polymorpha. Nos résultats mettent en évidence que le type d'interaction peut varier le long d'un gradient environnemental et que la présence de plusieurs stresseurs peut devancer un point de bascule. Cette thèse de doctorat contribue à l'amélioration de notre compréhension théorique des concepts d'interactions entre stresseurs et les points de bascule et combine, pour une des premières fois à notre connaissance, les deux concepts écologiques à différents échelles biologiques (individu, communauté, écosystème) Ce projet, qui s'intéresse tant à la portion d'eau douce qu'à la portion marine du Saint-Laurent, apportera également des informations importantes quant à la capacité de tolérance des écosystèmes face à l'empreinte humaine. En intégrant plusieurs composantes de l'écosystème, nous souhaitons mettre en lumière que l'évaluation de l'empreinte humaine sur les écosystèmes aquatiques doit être effectuée en intégrant plusieurs composantes afin de bien comprendre la complexité derrière le concept d'interactions entre stresseurs et de points de bascule. Ultimement, en apportant des informations et pistes de solutions quant à la vulnérabilité des communautés benthiques à plusieurs facteurs et intensités de stresseurs, ce projet contribuera à une meilleure gestion écosystémique des communautés benthiques du Saint-Laurent. / The coastal regions of estuaries are heavily affected by terrestrial and aquatic anthropogenic activities such as agriculture, fisheries, aquaculture and shipping. These activities lead to a diversity of environmental stressors that affect the structure, dynamics and functioning of benthic communities and will have impacts at different scales of the biological hierarchy, from the individual to the community. With increasing human activities, two main scenarios have been observed: i) the spatio-temporal overlap of multiple stressors, and ii) the intensification of single stessors. First of all, the spatio-temporal superposition of multiple stressors can lead to their interactions, where the presence of one stressor will accentuate (i.e. synergy) or attenuate (i.e. antagonism) the effect of another. Second of all, the intensification of single stressors can lead to tipping points, corresponding to substantial reorganizations in the structure and functioning of an ecosystem. Stressor interactions and tipping points have profound implications on ecosystems: often very difficult to predict, they can cause significant changes within communities, to such an extent that a return to initial states is sometimes impossible. These two concepts have been widely studied individually. However, very few studies have attempted to combine both concepts. The justification to carry out this doctoral project resides in the need to address the challenge of the increase of human activities and their impacts of aquatic ecosystems, particularly those in the estuary and gulf of the St. Lawrence River. More specifically, the objective of this thesis is to understand the effect of multiple anthropogenic stressors on marine and freshwater ecosystems by integrating multiple biological compartments. The thesis is structured around two main objectives divided into five chapters. The first objective is to characterize the scenarios of interaction between multiple stressors and their effects on benthic communities using experimental approaches in the laboratory or in situ (Chapters 1 and 2). The second objective is to identify tipping points when communities are exposed to different intensities and combinations of stressors using a literature review and a laboratory experiment (Chapters 3, 4 and 5). The objective of my first chapter is to characterize the individual effects of nutrient enrichment, salinity variation and increased water temperature on different biological responses measured at the individual level in two bivalve species (Mytilus spp. and Limecola balthica) and on microphytobenthos. We carried out a laboratory experiment with two exposure times (one and three months) and evaluated mortality levels, energy content and mineralization levels in Mytilus spp. and L. balthica as well as microphytobenthic pigments. Salinity variation and nutrient enrichment, individually and combined, had significant effects on mortality, shell mineralization, tissue energy content and sediment chlorophyll a concentration and more interestingly, their effect varied through time. On the contrary, increased water temperature had no effects on the investigated responses. These results suggest a prevalence of antagonistic interactions for the evaluated responses and the importance of considering the effect of single and combined stressors over time. Similarly, my second chapter has for objective to characterize in situ the effect of heat waves and nutrient enrichment on mussel beds and their associated ecosystem. More specifically, we carried out a three months experiment where mortality and energy content in Mytilus spp, microbial activity and sediment chlorophyll a and phaeopigment concentration were assessed multiple times over the duration of the experiment. Nutrient enrichment and heat waves, individually and in combination, affected tissue energy content in Mytilus spp., phaeopigments and the chlorophyll a/phaeopigments ratio. Regarding the other responses, no significant effect was detected, suggesting that stressors influence the compartments differently. This chapter demonstrates that the individual and combined effect of environmental stressors differ over time and illustrates the importance of combining several experimental approaches to understand the complexity behind the interaction between multiple stressors. My third chapter seeks, through a literature review, to (1) develop a historical perspective of tipping point studies in terrestrial, freshwater and marine ecological systems; (2) portray the research effort in different aquatic habitats; and (3) explore the results of experimental studies focusing on tipping points measured at the individual, communities, ecosystem level (incl. ecosystem functions and services) in a context of single and multiple stressors. The number of studies mentioning tipping points increases every year, but surprisingly very few have had to date as specific objective that to actually identify them (32.6%). Even fewer studies examine how adding an additional stressor may alter a tipping point. Also, several studies are interested in multiple responses, but only 25 % of these focus on multiple biological scales. These results allowed us to identify some shortcoming in the field and some potential solutions in order to renew this concept in a context of multiple stressors. In my fourth and fifth chapter, we conducted an experiment to study the effect of pulses of saltwater intrusion along a nutrient enrichment gradient on a freshwater community composed of periphyton, heterotrophic microorganisms, and the zebra mussel Dreissena polymorpha. We sought to (1) characterize individual and combined effect of the two stressors on the investigated responses; (2) identify a tipping point along the nutrient enrichment gradient in the absence and presence of pulses of saltwater intrusion and; (3) identify the effect of osmotic stress on the presence, timing or intensity of a tipping point. We evaluated mortality levels (chapter 4) and metabolic rates in D. polymorpha, microbial activity and periphyton biomass (chapter 5). Both stressors had individual and combined effects on all response variables, and we observed a change in the type of interaction along the gradient. The presence of osmotic stress along the gradient had for effect that to create a tipping point (metabolic rate) or to create an earlier tipping point (mortality). Our results highlight that the type of interaction can vary along an environmental gradient and the presence of multiple stressors may create of outpace a tipping point. In general, this doctoral thesis contributes to improving our theoretical critical understanding of the concepts of stressors' interactions and tipping points, individually and, for the first time to our knowledge, combined and this, considering multiple levels of biological compartments (individual, communities, ecosystems). More specifically, this project focuses on both the freshwater and marine portion of the St. Lawrence River, and provides essential information on the tolerance of ecosystems within the context of an increasing human footprint. By integrating several components of the ecosystem, we wish to highlight that the human impact on aquatic ecosystems must be assessed by integrating several components to fully understand the complexity behind the concept of stressors' interactions and tipping points. Ultimately, by providing information and possible solutions regarding the vulnerability of benthic communities to multiple drivers and intensities of stressors, this project will contribute to a better ecosystem management of benthic communities in the St. Lawrence.

Oxygen uptake rates by fine benthic organic matter were measured on samples from two Appalachian mountain streams to determine the effects of land use patterns on microbial respiration. Both Coweeta Creek in western North Carolina, and Wolf Creek in western Virginia, flow through national forest, agricultural land and scattered residential areas. Seven sites were sampled at six to ten week intervals over one year. Land use was determined using infrared aerial photographs. ARC/INFO was used to obtain land use areas for various land uses. Oxygen uptake rates were measured using a Gilson differential respirometer, at ambient temperature and 20°C. Temperature was the most important factor influencing oxygen uptake rates in both streams, with oxygen uptake rates consistently lower at ambient temperatures than 20°C. Oxygen uptake rates were higher in Wolf Creek than Coweeta Creek and were higher at sites that drained agricultural areas. Nitrates increased as the percent of agriculture increased in Wolf Creek, but not in Coweeta Creek. Nitrates were positively correlated with oxygen uptake / Master of Science